Cardiac hypertrophic remodeling underlies a large component of the morbidity and mortality of heart disease. It affects nearly 10% of the world's population given the high prevalence of hypertension and hypertrophy that evolves with it. We recently discovered that inhibitors of the phosphodiesterase PDE5a such as sildenafil, drugs widely used to treat erectile dysfunction, have potent effects on cardiac function and stress-remodeling. These and other new data supporting cardiac benefits have raised substantial interest for using these drugs to clinically treat forms of heart disease. However, remarkably little is known about how they are working particularly in the relevant setting where there disease is already established. At the primary level, inhibiting PDE5a increases the cyclic nucleotide cGMP, that can influence the heart directly, or active protein kinase G which then influences multiple proteins to modify the stress response. The cGMP/PKG system functions much like a brake, having little basal impact, but blunting cardiac stimulation by catecholamines or pathologic stress. Yet, PDE5a inhibition (PDE5a-I) appears to change cGMP levels little, while enhancing PKG activity and has effects that are quite different from other ways of enhancing cGMP/PKG (such as natriuretic peptide stimulation). New data suggests a prominent role of PDE5a-inhibition in suppressing activated G1q pathways via regulator of G-coupled signaling 2 (RGS2) and potentially canonical transient receptor potential (TRPC) channels. The mechanisms for these interactions, how they change as hypertrophic disease becomes established, and why chronic PDE5a-inhibition improves cardiac function while suppressing hypertrophy are unknown. The research in this proposal aims to provide this critical information in three aims, with studies conducted largely in mouse models, using aortic-banding pressure-overload to stimulate hypertrophy/remodeling. The first will determine how PDE5a-I acutely improves cardiac function and how this is altered by chronic hypertrophic disease. The second hones in our finding that PDE5a is post- translationally modified with chronic hypertrophy, altering activity and cellular localization less than expression, but that this impacts its stress modulation. We will identify mechanisms for this key regulation. The final aim tests the role of PKG activation and suppression of G1q-coupled signaling for both improved cardiac function and anti-hypertrophic effects in pressure-overloaded hearts. The successful completion of these studies will greatly expand our understanding of how PDE5a-I modulates normal and diseased hearts, and inform clinical trials testing such drugs for treating heart disease. RELEVENCE: Nearly 10% of the world's population develops an increase in muscle mass (hypertrophy) of their heart which increases their risk of suffering from heart disease. We discovered that sildenafil (Viagra), a drug that blocks the enzyme PDE5a and is widely used to treat erectile dysfunction, may also suppress cardiac stress-responses. This project will determine how sildenafil is working and the pathways that are involved, and how this may change in normal as opposed to diseased hearts.